1. Field of the Invention
The invention disclosed relates to arc resistant electrical distribution equipment.
2. Discussion of the Related Art
Circuit breakers are used in motor control units, wherein the motor control unit may be inserted or removed from a motor control center (MCC) switchgear cabinet without shutting down all power to the cabinet. The motor control units have a connected position in which female connectors of the breakers are connected to respective bus bars, and a disconnected position in which the female connectors are disconnected from the bus bars. To cover access to, and prevent inadvertent contact with, the bus bars when the motor control unit is in the disconnected position, a common shutter system has been utilized across all power phases supplying the motor control unit.
Motor control units may have the requirement of making and breaking connections with installed bus bars or bus bar extensions i.e. the incoming power conductors contained in the MCC. The making and breaking of connections and the handling of heavy currents in the area of connection between control devices like circuit interrupters (breakers), makes this area of the cabinet particularly susceptible to arcing. An arc flash is the rapid release of energy due to an arcing fault between phases, neutral or ground contacts. The resulting arc flash has the potential to cause considerable damage, including arcing-induced erosion of the contacts and injury to operators. The temperature of an arc flash may be capable of vaporizing metal and sending a blast of plasma and molten metal in all directions with extreme force. Damage may be caused to the switchgear both by the explosion of the arc flash and by the heat radiating from the blast. It is important to minimize the potential for harm to equipment and people by containing and redirecting the arc energy out from the switchgear and away from personnel.
Arc resistance pertains to the ability to withstand the destructive energy released during an arc flash, by interrupting and channeling the energy away from personnel and adjacent equipment. Passive arc resistance may include directed venting of the arc flash energy and gases out of the switchgear and reinforcement of the switchgear structure to withstand the blast. An example of passive arc resistance from the applicant features so-called arc-block technology which provides capability to passively attenuate and extinguish arc events, and which is described in the co-pending US Patent Application Ser. No. 13/452,145, filed Apr. 20, 2012, entitled “Passive Arc Management System With Flue Chamber”, of common ownership herewith. The co-pending application describes an electrical distribution cabinet has an arc attenuating chamber surrounding the electrical connection point between a cluster, i.e. electrical power connector, of a draw out circuit breaker and a bus bar extension. The arc attenuating chamber is formed by sliding a cluster shield surrounding a cluster at the back of the breaker, into a slightly larger arc attenuating box that surrounds the bus bar extension, so that the leading edges of the cluster shield and arc attenuating box overlap and form the chamber. The arc attenuating chamber provides a flue channel that lengthens the arc and attenuates the current and temperature until the arc is extinguished.